How To check a motor to ground

[reefbum11]

I am new to building engineering and we have a motor on our air handler 3 phase 15 horse motor and we need to check it for ground how do i do that. thanks for the help.

 

[Skogsgurra]

Do you need to check its ground (PE) connection? Or are you checking for a short to ground?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[aolalde]

Disconnect all the power leads in the motor connection box. Check that all have ID's to reconnect properly.
Insulation resistance to Ground: Tie together making electric connection between all motor terminals. Connect a"Megger Tester" hot terminal to the common connection and the return to the motor frame. Aply 500 Volts DC test and allow for 1 minute, record the megohm reading and the winding temperature. For accurete evaluation you could refer the insulation resistance to 40 °C. Check IEEE Std 43.

 

[DickDV]

aolalde's test is just right for 230V motors. For 460V motors be sure to use a 1000V megger.

 

[aolalde]

Hi DickDV.
IEEE Std43-2000 part 5.3 Table 1 states the guidelines for DC voltages to be applied during insulation resistance test.
Winding Rated Voltage Voltage Test
<1000 500
1000-2500 500-1000
2501-5000 1000-2500
5001-12000 2500-5000
>12000 5000-10000


I prefer to follow a recognized standard rather than popular rules of thumb.

 

[DickDV]

You may do as you please, aolalde, but, as for me and thousands of EASA motor shop techs across the US, if makes little sense to test insulation integrity at 500VDC when the AC service voltage reaches over 1400 voltage twice each cycle (1000 * 1.414).

 

[electricpete]

I will have to agree with aolalde on that one. In the context of 460 volt motors, 500vdc is specified for insulation resistance testing by IEEE43-2000.

And since you mention EASA, EASA AR100 (specification for rewind/repair) section 4 also specifies 500vdc.

1000 vdc on a 460vac motor would be considered a hi-pot in my book. Most likely would pass... but I'd hate to be the test guy that failed a motor applying more than standards specify, when owner of the motor simply asked to check for ground... didn't ask for hi-pot.

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[electricpete]

and no, I'm not saying 1kv is THE hi-pot voltage for 460 vac motors. But it falls above the range of standard test voltages and is a hi-pot in that sense.

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[edison123]

For a motor or generator of any voltage which comes to my shop, I start with 500 V megger to be on the safe side. If the 500 V values are ok, then I upgrade to applicable higher voltage.


For lv motors (< 600 V) rewound at my shop, I use 1 KV megger.



*Why a man thinks he outrun a chasing dog when it has twice as many legs?*

 

[ScottyUK]

In the UK on a standard 415V machine I would go with 500V windings to ground, 1000V between windings (if they were separable). This is the same test as for power cables at this voltage.


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Sometimes I only open my mouth to swap feet...

 

[electricpete]

edison - the fact that you checked it at the lower voltage first indicates the first check (500vdc) was an insulation resistance check and the second check (1000vdc) was a hi-pot. You wouldn't go straight to 1kv.

Scotty - I hadn't heard of the higher phase-to-phase test voltage.. But since the subject is checking for grounds... 500vdc matches the IEEE43.

To go back to the beginning of my comment. The subject here is insulation resistance check ("checking for grounds").

There are opinions and there are standards. As a tester, if you go above the standard or specification test voltage when a hi-pot was not requested, and the motor fails, you are not in a good position. You might say that in your opinion the the motor was bad anyway, but you don't have a standard to fall back on. Why would anyone put themselves in that position intentionally?

dickdv said he was speaking for thousands of EASA repair shops. In fact the document I cited is an EASA document:

http://www.easa.com/indus/ar100-2001.pdf

4.2.2 - Insulation Resistance Test

Winding rated < 1000 v => Insulation Resistance test voltage 500 vdc

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[edison123]

Hi pete

I did mention that I "upgrade to applicable higher voltage" which depends on the motor/generator voltage. So, if it is a LV machine with someone else's winding (that has come in for a overahul and check), I would keep it at 500 V.

(In any case I wouldn't call a 1000 V megger test on a LV AC machine as a hipot. If the winding fails at 1 KV DC, it is unfit for service anyway. That is why I do 1 KV megger test before and after a 2 KV AC hipot and 2 KV surge on the machines rewound in my shop. For overhauled LV machines, I use 1.5 KV AC and 1.5 HV surge. If they fail these tests, then it is better that they fail in my shop than during service )

BTW, I do not know whether our replies satisfy reefbum11's requirement.

*Why a man thinks he outrun a chasing dog when it has twice as many legs?*

 

[DickDV]

Electricpete, with all respect, sir, my statement was "thousands of EASA motor shop techs", not "thousands of EASA motor shops".

As usual, the guys that actually do the work often have to add a little judgement to what they do for their customers.

 

[fatherjoe]

I need a little help............

I've just got back into working with motors, after spending to much time in digital world.

I need to check out motors periodically, and have been doing the following:

1) Check winding continuity
2) Check winding continuity to ground. Redundant but easy, if I'm going to Megger the motor
3) Check winding w/ insulation tester. I've got access to a Fluke 1587.

Proceedurally with a 3phase motor, can I test the motor leads seperately or should I gang them?

Should I expect different results for Delta and Y motors based testing?

Thanks in advance.

 

[Zogzog]

2005 NETA MTS refers to IEEE43-2000. Thousands of certified NETA techs in North and South america have been following this standard since 1972.

I am with aolalde and EP, you should follow a recognized standard.

 

[Zogzog]

From the EASA website "The following organizations produce documents and standards, some of which are referenced in the EASA Recommended Practice for the Repair of Rotating Electrical Apparatus."

Listed below that statement, IEEE. So I wonder if that book references the IEEE 43?